The present invention relates to a method of controlling the speed reduction ratio in a continuously variable speed transmission for use in a motor vehicle.
One known method of controlling the speed reduction ratio of a continuously variable speed transmission mounted on a motor vehicle to control the running of the motor vehicle is disclosed, for example, in Japanese Laid-Open Patent Publication No. 62(1987)-237164. According to the known control method, a reference engine speed is preset according to an opening of a throttle valve of the engine of the motor vehicle, and the rotational speed of the engine is controlled so as to match with the reference engine speed.
In controlling the engine speed to cause the engine speed to correspond with the reference engine speed, a speed reduction ratio is controlled to be as small as possible (toward a top position). This controlling method yields improved fuel economy and a lower engine speed for the same vehicle speed, which in turn allows the vehicle to run quietly.
In order to increase the engine speed when the accelerator pedal of the motor vehicle is depressed for acceleration, a rate of change of the speed reduction ratio, which corresponds to the reciprocal (1/V) of the vehicle speed is required (as described in detail later on). Therefore, when accelerating the vehicle at a lower vehicle speed, the increase in the engine speed tends to be slower than when accelerating the vehicle at a higher vehicle speed, with the result that the engine is apt to suffer knocking, and the power available from the transmission is not large enough.